


Sounds kind of Fishy

by rehliamonster



Series: Prostitute Sans Series [4]
Category: Undertale (Video Game)
Genre: Banter, Casual Sex, Cunnilingus, Ecto-Penis (Undertale), Ecto-Tongue (Undertale), Ecto-Vagina (Undertale), Ectobiology, Ectoplasm, F/M, Fingerfucking, Gentle Sex, Light Dom/sub, Multiple Orgasms, Oral Sex, Other, Overstimulation, Plot What Plot/Porn Without Plot, Prostitution, Teasing, Tribadism
Language: English
Status: Completed
Published: 2018-06-23
Updated: 2018-06-23
Packaged: 2019-05-27 12:30:45
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,096
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/15024656
Author URL: https://archiveofourown.org/users/rehliamonster/pseuds/rehliamonster
Summary: Undyne somehow didn't expect Sans to show up when she got a referral for a prostitute who would just lie there and take it.They still have fun together, and Sans still loves his job.





	Sounds kind of Fishy

**Author's Note:**

  * For [captain_shitpost](https://archiveofourown.org/users/captain_shitpost/gifts).



> Posted to make my sick buddy feel better :3 get well soon, cap!

“Well, this is awkward,” Undyne said, closing the door behind Sans so nobody could hear their conversation. Then she rubbed a finger against the point between her eyebrows, massaging the building crease there. “What the hell. _You_ , of all people?”

“i don't mind,” Sans shrugged. “but i can leave if you don't want go through with it.”

“Why are you even doing this?!”

“had some debts to settle. more than my salaries could take care of, heh. heard this was an option and figured i’d try. i mean, you know better than anyone that bein’ paid for lying back and doin’ nothing is my dream job,” he grinned. “had so much fun doin’ it that i kept going even after the debts were gone.”

“I know someone who's really relaxed, they said,” Undyne groaned. “Lies back and takes it, they said. I should've known they meant you before they sent you over. How embarrassing!”

“hey, you know the rules, nobody will hear a peep about this,” Sans reminded her. “we can pretend this never happened if you're that uncomfortable.”

“Yeah, yeah, I know,” she grumbled, though Sans noticed that she didn’t tell him to get lost yet. “So, uh. The person who recommended you. They said the person they knew was… creative with their junk. Thought you were a guy.” 

She looked at him with open curiosity now, in spite of her still evident awkwardness. 

“i’m a skeleton,” Sans shrugged, having no particular trouble answering this. “whatever we make is ectoplasmic. ‘s flexible that way. i can make anything. cock, cunt, both, tentacles, flesh in general, you name it i got it.” Would it be too much to throw a wink her way? Nah. It was funny. Wink. There he went. 

She snorted, the curiosity and amusement slowly breaking through the initial embarrassment. 

“Don’t look at me like that, you nerd!”

“that don’t getcha in the mood?” he chuckled. 

Instead of answering, she looked him up and down. Took him in, in a way that was both similar and completely different to how she had sized him up when she first hired him as a sentry. Both times she wanted to see what his body and magic could do, but the intent behind it now was something else entirely. It was enough to send a shiver down his spine, although he did his best to suppress it.

“So you think we can do this without making it weird?” she asked him. 

“i got no shame, as people like to remind me,” he grinned. But then he got serious, because some things even he knew he shouldn’t take lightly. “but yeah. we both know we won’t talk about it ‘n as long as you’re good with my additional rules i’m good for it if you wanna.”

“Let’s hear your rules then,” Undyne demanded. 

Sans didn’t mention what was already clear to both of them: no prostitute and no client ever talked about who bought and sold what. Referrals only, and you either knew how to get what you needed or a matching referral, or you were out of luck. Anyone who outed a client or prostitute would be shunned in turn, by the entire Underground, for breaking the trust. The entire population was at least vaguely aware of the happenings and with how tight living space was, with everyone pretty much knowing everyone else, looking the other way and not talking about what you knew was the only way to guarantee some privacy. Anyone who didn’t respect that deserved the cold treatment they would get as a result. 

“kay, first of all,” Sans began, “papyrus won’t know a thing. not even through a referral. i wanna keep this to myself only, so he’s not gonna hear anything about what i’m doing no matter what.”

“Pfff, yeah, I wouldn’t wanna share my sexcapades with my siblings either,” Undyne cackled, and Sans could’ve kissed her just for that already, that she didn’t immediately assume that this rule had anything to do with Papyrus being too naive or whatever, like some other monsters did. Then again, she actually knew his brother. 

“cool. secondly, no pain, nothing dangerous. don’t care if you tease me or edge me or tie me up or whatever, but i gotta look after myself,” he continued. 

“Of course!” Undyne growled, crossing her arms in front of her. If anything, she looked approving that he was taking measures to ensure his safety. He expected as much of her. 

It was the next part that could be tricky. 

He looked her dead in the eye. Met the challenge in it with one of his own, because that was generally how you got anywhere with Undyne. 

“third. i ain’t gonna do any of the work. when they said ‘lies there and takes it’ they meant it. i can top or bottom, whichever, but you do the work, not me.” 

The sentence hung in the air between them. He knew this reminded her of his laziness at his sentry stations, and not in a good way necessarily. Sans could see the twitch under Undyne’s eye, the vein popping at her temple. He braced himself. 

Undyne threw her head back and broke out into loud, genuinely amused laughter. 

“Seriously?! You lazy little fucker!” 

“heh. what, did ya expect somethin’ different with me?” 

“Fuhuhuh. No, not really. Fine, I do the work huh?” She gave him her best feral grin and flexed. “I can do that!!” 

“so we got a deal?” he confirmed. 

“Yeah!! Let’s see what you got!” 

Heh. This should be interesting, Sans thought. He never thought he’d end up fucking another one of his bosses, and yet here he was. Out of everyone in the Underground he was probably among the last monsters Undyne ever expected to end up with too. Life could be funny that way. 

She paid the price he named for what she had in mind without fuss, and then took him to her bedroom. Sans had seen quite a few bedrooms in this line of work, so he got pretty good at guesstimating what kind of bedroom a monster would have. Hers still surprised him a little. It was very calm, a dark floor, neutral walls and different shades of soft blue on her bedsheets, a single water damaged picture of a beach on the surface hanging on the wall. That was all. Based on her personality he would have expected brighter colours, maybe another doily, anime posters since she recently got into that, maybe even some weapons. But no, it was incredibly simple and soothing. He actually quite liked it himself. 

In the time he had admired the scenery, Undyne had already completely undressed herself. 

“uh, whoa,” he blurted out. 

“Impressed?” she asked him with a cocky grin, flexing again. 

“yeah,” he admitted. No need to beat around the bush. She was rail thin of course, but her entire body was nothing but tightly packed muscle covered in slick, shiny scales. She looked wiry and strong and confident and her long red hair framed her torso like a halo. It was quite the sight. She seemed to like his confession as her grin widened further at his single word. 

“Let’s see you then!” she challenged him, putting her hands on her hips. 

He nodded and slipped out of his hoodie. Since Undyne hadn’t taken any care with her clothes and had just dropped them on the floor, he did the same. He pulled his shirt off and stepped out of his shorts, standing in front of her bare-boned. 

She looked surprised somehow as she took in his naked form. 

“Huh. You look smaller without all that stuff covering you,” she observed. 

He wanted to make a clever quip about being a skeleton and something about not having a body, but his train of thought derailed when she stepped closer and trailed her fingers over his ribs, taking care not to hurt him with the sharp tips of her fingernails. He sucked in a sharp breath at the sudden touch. 

“Smooth,” she mumbled. “That’s really cool.” 

She poked her fingers through the gap between two ribs and he hissed. 

“shit, undyne…”

“Oh, uh. Too much?” She withdrew immediately, holding her hands up. 

“n-no, just… that’s a sensitive spot. wasn’t prepared, heh.” He tried to play it cool but the sudden stimulation had unbalanced him a little. 

“You’re blushing,” she pointed out with a fiendish grin. He felt his skull heat up even more in response. “Fuhuhuh. How cute of you. I like that.” 

“uh. good.” Geez. Now it was his turn to feel a little awkward, if he was being honest. 

“Come up here,” Undyne said, patting the cover of her bed. He was glad for the instruction, to move things towards more well-known territory. He climbed up and allowed her to push him back until he was lying flat on his back, comfortable on her soft blanket. 

Undyne remained seated, but began running her fingers over his body again, indulging in her curiosity. She took her time too, tried out each bone and smiled at every shuddering and hitching breath he didn’t manage to keep in. She seemed less interested in having him touch her in return, appearing perfectly satisfied to slowly explore him for now. 

It didn’t take long before his entire body was flushed in soft cyan, heated and prickling with the need to manifest something and get on with it. 

“Make a body,” she suddenly demanded, before he could try beg her to have mercy on him. He gladly complied, allowing his overagitated magic to collect on his pubis and from there onwards; over his femurs, the hollow cavity between his pelvis and his ribcage, over his ribs up to his collar bones and down his humeri. 

She had specifically demanded a vagina and breasts but otherwise not specified anything about the size or shape, so he went with what came naturally to him. His magic tended to err on the chubbier side of things if he let it do whatever, which he felt fit him. His breasts swelled to a size that was more than just a handful, his belly had a curve to it, his thighs were rubbing against each other, his arms were a little flabby. His cunt was plump and already wet. It was a big contrast to Undyne’s body, wiry muscle and tight flesh contrasting with his soft fat. 

“Oh,” she sighed, running a hand over the newly formed body he had made for her. “Oh, this is…” 

She looked surprised again, but in a good way, so he relaxed. He could tell she liked this. 

Her hands wandered over his stomach and up to his breasts cupping them and testing their weight. Despite the fact that they looked large on him, Undyne could encompass one in her hand almost completely. Her scales felt warm and smooth against his ectoplasmic flesh. Then she leaned forwards, pressing her full lips onto the very tip of his nipple. 

“hnngh…”

Her lips were really soft. Her mouth parted and Sans briefly saw the glint of her terrifying teeth, but she was careful not to hurt him. Her tongue darted out and licked over the nipple she had just kissed, playing with it until it hardened. Then she enclosed it completely and sucked on it. 

“fffh... “

The wet heat of her mouth felt amazing on his sensitive flesh. The suction sent shivers through him, an electric sensation building inside of him. More and more of his breast vanished into her mouth until she could take no more and he barely felt the tips of her teeth scrape over his simulated skin. When she pulled back again, a trail of saliva left her connected to him for a brief moment before it snapped. His breast was wet and flushed a darker shade of blue where she had sucked it. She put her fingers onto the wet spot and rolled the nipple between two of them. 

“ah…”

“It's nice how vocal you are,” Undyne commented, leaving him no time to reply before she leaned forwards again to lavish the same kind of attention in his other breast. 

He held his breath when he felt her free hand stroke over his thigh, gently caressing the sensitive inside. It exploded out of him when she trailed her fingers higher, getting closer and closer to the point where his legs met. 

“p-please,” he begged when she stopped there briefly without moving. 

“Not yet,” she mumbled, removing her mouth from his breast to lick the valley between. “Gotta practice my moves.”

He almost wanted to laugh at that statement, but she chose that exact moment to graze his clit with the lightest touch of her finger and it came out as a breathless moan instead. 

He heard her hum while she pressed a trail of kisses over his stomach, sucking on the flesh here and there. He was panting heavily by now, his cunt slick from pent up lust. He wanted to be touched. 

As if she had heard him, she took the final plunge and moved her head between his legs her lips hovering over his wet entrance. Her breath was hot on his sensitive flesh and his spine arched when she finally pressed her mouth onto him.

“ah fuck…!”

He felt her chuckle against him more than he heard it, a faint vibration as she tasted him for the first time. She kissed his pussy languidly before she delved into his opening with her tongue. 

Sans’ mouth hung open, a low moan stuck in his throat as he savoured the feeling of Undyne’s tongue exploring the sensitive surface of his inner walls, gliding over every inch she could find. It was an almost ticklish sensation that was nonetheless intense, leaving him shivering and shaking as the heat built in him. 

His phalanges gripped her bedsheets hard as she ate him out, shamelessly moaning and sucking up his juices. 

Her hands were on his thighs, petting and kneading the plump flesh there. Every movement of her hands was slow and deliberate. 

He hadn't expected her to be so gentle with him. 

“oh shit… yeah… shit….undyne…” 

His breath came in shorter and shorter gasps as he closed in on his orgasm, a whine building underneath his babbles as some part of him wondered if she'd let him come or not, preparing in case she didn't. 

But Undyne only redoubled her efforts, licking over the whole of his cunt before she concentrated on sucking on his clit and the pleasant surprise of it was enough to release the tension that had been building in him. 

Sans came with a trembling cry, drawn out and softer than he usually sounded. 

He twitched against Undyne’s mouth and she let him, licking and kissing him further to draw out his pleasure. 

A final, pleased sigh made it out of his mouth when she stopped and his spine reconnected with the bedsheets. Glancing down, he found her staring up at him with a catlike grin, licking her lips and teeth. He could see the powdery blue of his own slick there. 

“You taste really nice,”she practically purred at him. 

“thanks,” he laughed. He felt breathless and warm. “you’re pretty good at this.”

“If you say so, you're the expert,” she cackled. “You good for another go?”

“shit, yeah,” he agreed immediately, curious to see where else she would take this. 

Sans was used to his clients going all out with their desires when they booked him, mostly focused on satisfying themselves. Which was fine, they paid for it after all. But Undyne had surprised him by being nothing but attentive and gentle with him so far, and considering she had mentioned wanting to practice her moves… 

She laughed as she climbed onto him, seating herself on his pelvis. It took her a bit of maneuvering before she got the angle right, but then Sans felt her pussy rub against his clit and hissed. 

“Damn you're still wet,” she laughed, rolling her hips forwards. “Makes for a nice glide.” 

He couldn't reply, already moaning again as her cunt slid over his, the electrifying tingles of overstimulation washing over his lower body. 

Undyne’s breathing was growing rougher now too, the drag of her cunt against his clearly affecting her after she neglected her needs while she ate him out. He could feel her muscles twitch against him with every slow downwards grind she performed, and his pussy throbbed in response. 

She was bracing herself on his stomach now, looking down at him through her single lidded eye. Her expression was beginning to glaze over, a deeper flush spreading on her face. It seemed that a part of her was far away and Sans wondered who or what she imagined as she moved faster, chasing her own release on his body. 

He reached up with his hands to slide them over her thighs and her waist and for the first time, he heard a whimper between the rougher pants and moans that made up the majority of her vocalisations so far. He could feel each of her muscles as he touched her, every shift of them as the tensed and relaxed with her rhythmic motions. 

“Give me your hand,” she suddenly groaned, already grabbing his wrist before he even nodded. Once she saw his agreement, she unceremoniously brought his hand to her front, inserting three fingers into her without preamble. 

“fuck,” he groaned, sockets wide as he took in the side of it, his fingers spreading the glistening lips of her pussy. She felt hot and soft. His fingers curled automatically and her head fell back, a loud and sharp groan filling the room.

She began to move faster, riding his fingers hard. A slapping sound accompanied her movements. Her hair fell in wild strands over her shoulders, some sticking to her breasts and waist. Not from sweat though - despite her rapid pace and her ragged moans she didn't seem tired at all. He doubted she'd be winded even if she kept this up all night and couldn't help but admire her stamina. She looked like a warrior goddess straight out of a legend. 

Her hips shuddered forwards at her next downwards grind, pressing her clit against his thumb. 

“Ngah…” 

She was tensing up. 

“fuck yeah. take what you want,” he encouraged her, his voice deep with desire and admiration. 

Her hips snapped down with his words and she stayed there, trembling as she was pressed against him. Her mouth was wide open but no sound escaped. 

Undyne came on his hand in complete silence. 

His thumb twitched against her clit and only then did she groan, deep and guttural, before falling forwards on top of him. Her upper body felt hard against his conjured chubby flesh, an interesting change in comparison to how things usually were for him. She was breathing against the vertebrae of his neck, deep and slow. 

He let her rest there while she came down from her high, although he got the impression that she didn't really need the break for physical reasons. It was more that she was savouring the afterglow. His cunt throbbed now that he wasn't so focused on her, the stimulation she had given him before she focused on her own pleasure after all making itself known. 

Before long, her hands were back on his body though, playing with the small rolls of his fat. 

“You're a lot better at this than at being a sentry,” Undyne said against the crook of his neck, making him laugh. 

“told ya lying back and and doin’ nothing is the perfect job for me,” he chuckled and this time she joined him. She pushed herself back up enough to look at his face, only a short distance away from him while her body remained pressed to his. 

“You said you can make whatever.” It was half statement and half question, accompanied by a calculating look in her eye. 

“yup,” he confirmed. 

“Make a tongue!” 

“ok.”

That was easy. Took him less than a second before he could stick it out at her with a lazy wink. She reached up and took it between two fingers, pulling at it a little as she looked at it. Experimentally, she licked over it with her own tongue. He could still taste himself on her. 

“Hnnn.” She let go of his tongue and instead began kissing him properly, rubbing her body against his. 

It occurred to him only then, when she began moving once more, that his fingers were still buried in her cunt. 

“I'd ask you to wear a dildo for me, she said suddenly, stopping the kiss. “But you can just make a cock.”

“whichever you prefer,” he shrugged. 

A thoughtful expression passed over her face before she decided. 

“Make one.”

“ok. any special wishes?”

“Let's save the surprises for next time.”

He couldn't help but give her a cheeky grin at the implication that she might want to hire him again. 

Changing his genitals now took him a bit longer than the tongue. The fact that his magic was already manifested in a still mildly pulsing pussy made it harder to switch it around, requiring more concentration before his ectoplasm complied and transformed his clit into an already hard dick. It prodded against Undyne’s entrance and he groaned at the wet sensation against the sensitive head.

“Needy,” Undyne teased him, laughing when she pressed back slightly and felt his cock twitch in return. 

“w-well it's… ahn…” 

His stuttered and incomplete attempt at arguing back actually caused Undyne to giggle, a sound he never expected her to make. She immediately clamped her mouth shut and glared at him. 

“You didn't hear that!” 

“heh. heh heheh.”

“Sans!”

“begging already? oh yeah, say my name more,” he snickered. 

“Fuhuhuh. A challenge I see. So you _can_ do more than just lying there and taking it.” Her grin was fiendish. 

Briefly, he wondered if he was going to regret his big mouth. 

Undyne kept the eye contact going as she swiftly moved backwards, spearing herself on his cock. She didn't even flinch. 

“undyne holy shit!” he squawked, overwhelmed by the sudden tight heat enveloping him. 

She cackled where she lied on top of him, deliberately squeezing her muscles so she tightened even further around him. Briefly she let go, and then squeezed again. 

“hhhhhhhh!”

“Anything to say?”

“p-please…”

“See who's begging now?”

“undyne…!”

“Yes.”

She tightened harder around him and grinned at his broken whine. Her golden eye was burning with intensity, the beautiful colour seemingly filling his field of vision completely. 

“You're lucky I'm not that mean, Sans,” she whispered against his teeth, pressing a kiss there before she rolled her hips against him, fucking him hard and fast while her walls kept squeezing him. 

He came almost immediately, whether because of the increased pace or because of the way she said his name, he wasn't sure. 

His limbs locked up as he rode out the sensation, a mixture between a grateful sob and exhilarated laughter on his teeth, before both bottomed out into a deep, resonating moan. 

Undyne kept squeezing him even after he was done, reaching down to rub her clit until she managed to eke out a shorter, smaller shudder that signalled her orgasm. It was intense after he already came, and he was almost glad when she slid off him to rest at his side. 

There was another moment where they were silent. 

“So,” she asked when they had caught their breath. Or rather, when she had waited for him to catch his breath. “What do you say?”

“huh?” 

“How was I!” she insisted impatiently. He found her blushing a bit when he glanced over and did his best not to laugh.

Right, practicing her moves. 

“i liked it a lot,” he told her. “you're very attentive.” 

He wanted to tell her how nice it was that she had been so gentle with him, but he knew that would only embarrass her. She wasn't good with admitting to the softer parts of herself. 

“Of course! That's how you do it!” she grinned. “I still wanna use a dildo on you.”

“still got time…”

“What! You're still good to go?!” she asked with a surprised cackle. 

“i mean, yeah? i’m used to taking a lot,” he explained with an easy grin. 

Uh oh. 

The challenging glint that built in her eye at hearing this promised that she would definitely take him up on that…

He couldn't wait.

**Works inspired by this one:**

  * [Prostitute Papyrus](https://archiveofourown.org/works/15029519) by [Kamari333](https://archiveofourown.org/users/Kamari333/pseuds/Kamari333)




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